Recently, as the height and size of buildings, structures, and the like has increased, steel used in such buildings and structures has increased in size as compared to the related art, and there has been demand for improved strength therein, and thus, the thickness of steel has gradually increased.
Although in order to manufacture large welded structures, higher levels of strength have been demanded in steel used therein, relatively low yield strength ratios are still demanded to improve shock resistance. In general, the microstructure of steel is commonly formed to have a soft phase like ferrite, and the yield strength ratio of steel is known to be reduced by implementing a structure in which a hard phase such as bainite, martensite, or the like is dispersed in a proper manner.
In order to weld high strength structural steel to manufacture welded structures, high efficiency welding is required. To this end, high efficiency welding having advantages in terms of construction cost reduction and welding procedure efficiency has commonly been used. However, in a case in which high efficiency welding is carried out, there is a problem in which crystal grains may grow or structures may coarsen during the welding process in a weld heat affected zone (positioned several millimeters from the interface between a welding metal and the steel in the direction of the steel) of a base metal, affected by heat, thus significantly reducing toughness.
In particular, since a coarse grain weld HAZ adjacent to a fusion boundary is heated to a temperature close to the melting point by welding heat input, crystal grains may grow. In addition, as an increase in the welding heat input slows down a cooling speed, coarse structures may be easily formed. Furthermore, since microstructures having difficulty in securing a sufficient degree of toughness, such as bainite, martensite-austenite, or the like, are formed in a cooling process, toughness in the weld HAZ in welding zones may easily be reduced.
In structural steel used in buildings, structures, or the like, not only high strength, but also a high degree of toughness is required in welding zones of steel for safety requirements. Therefore, in order to secure the stability of final welded structures, weld HAZ toughness needs to be secured, and in detail, microstructures of the HAZ, causing the deterioration of HAZ toughness, need to be controlled.
To this end, in Patent Document 1, technologies to secure toughness in welding zones through the miniaturization of ferrite using TiN precipitates are described.
In more detail, the content ratio of Ti/N is managed to form sufficient fine TiN precipitates, thus refining ferrite. Thus, when 100 kJ/cm of heat input is applied, structural steel having around 200 J of impact toughness at 0° C. may be provided.
However, since weld HAZ toughness is commonly relatively low as compared to steel having 300 J of toughness, there is a limitation in securing the reliability of steel structures through the large heat input welding of thickened steel. In addition, there is a problem in which production costs increase, in that a heating process prior to hot rolling may need to be performed twice in order to secure fine TiN precipitates.
If a weld HAZ has the same level of toughness as that of steel, stable and high efficiency welding on large thick steel, such as buildings, structures, or the like, may be performed. Thus, there is demand for the development of steel for a welded structure in which stability and reliability are secured in such a manner that the weld HAZ has a degree of toughness equal to or higher than that of steel.
Patent Document 1: Japanese Patent Laid-Open Publication No. 1999-140582